[PATCH] Fix multiple DNS queries when negative results returned

[Ricard Wanderlof]

We had a problem whereby multiple DNS queries were being made against the 
same host, even if a negative result was returned the first time. This 
patch attempts to fix this by keeping track of which hosts returned 
negative responses in the retry loop in__dns_lookup(). It can 
significantly reduce corresponding network traffic in large networks.

Patch made against svn 22530 and included as an attachment as well as 
inline for review.

/Ricard



Index: libc/inet/resolv.c
===================================================================
--- libc/inet/resolv.c	(revision 22530)
+++ libc/inet/resolv.c	(working copy)
@@ -757,6 +757,7 @@
  #ifdef __UCLIBC_HAS_IPV4__
  	struct sockaddr_in sa;
  #endif
+	bool negative_results[MAX_SEARCH + 1] = { 0 };

  	fd = -1;

@@ -812,6 +813,9 @@
  		__UCLIBC_MUTEX_UNLOCK(__resolv_lock);

  		DPRINTF("lookup name: %s\n", lookup);
+		/* Do not try the same lookup again. */
+		if (variant < (MAX_SEARCH + 1) && negative_results[variant + 1])
+			goto again;
  		q.dotted = (char *)lookup;
  		q.qtype = type;
  		q.qclass = C_IN; /* CLASS_IN */
@@ -924,6 +928,8 @@

  		if ((h.rcode) || (h.ancount < 1)) {
  			/* negative result, not present */
+			if (variant < (MAX_SEARCH + 1))
+				negative_results[variant + 1] = true;
  			goto again;
  		}

--
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30
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